1. Field of the Invention
This invention relates to abrasive products having a resinous binder which bonds abrasive granules to a backing sheet or to a fibrous sheet.
2. Discussion of the Prior Art
Coated abrasives generally comprise a flexible backing to which an adhesive or adhesives bond a coating of abrasive granules. The backing may be paper, cloth, film, vulcanized fiber, etc., or a combination of one or more of these materials, or treated versions thereof. The abrasive granules may be formed of flint, garnet, aluminum oxide, alumina zirconia, ceramic aluminum oxide, diamond, silicon carbide, etc. Popular binders include phenolic resins, hide glue, urea-formaldehyde, urethanes, epoxies, and varnish. Phenolic resins include those of the phenolaldehyde type.
The coated abrasive may employ a "make" coat of resinous binder material which is utilized to secure the ends of the abrasive granules onto the backing sheet as the granules are oriented and a "size" coat of resinous binder material over the make coat which provides for firm adherent bonding of the abrasive granules to the backing sheet. The binder of the size coat may be of the same material as the binder of the make coat, or it may be of a different material.
In the manufacture of coated abrasives, the make coat and abrasive granules are first applied to the backing, the make coat is partially cured, then the size coat is applied, and finally, the construction is fully cured. Generally, thermally curable binders provide coated abrasives having excellent properties, e.g. heat resistance. Thermally curable binders include phenolic resins, urea-formaldehyde resins, urethane resins, melamine resins, epoxy resins, and alkyd resins. In order to obtain the proper coating viscosities, solvent is added to these resins. With polyester or cellulose backings, however, curing temperatures are limited to about 130.degree. C. At this temperature, cure times are long. The long cure time along with the requirement of solvent removal necessitate the use of festoon curing areas. Disadvantages of festoon curing areas include the formation of defects at the suspension rods, inconsistent cure due to temperature variations in the large festoon ovens, sagging of the binder, wrinkling of very flexible webs, and shifting of abrasive granules. Furthermore, festoon curing areas require large amounts of space and enormous amounts of energy. If one could use a total drum thermal cure, i.e. wherein the coated abrasive is wound up in a roll or jumbo, and then placed in an oven, this would eliminate many of the problems associated with festoon curing. Drum curing does not require the use of a large oven; consequently, the amount of energy and space required is considerably less than with festoon ovens. However, it is not possible to use a drum cure by itself with the conventional thermally curable binders mentioned above, because the use of a festoon oven is initially required to remove the large quantities of solvent contained therein.
It has been proposed to use radiation curing processes to avoid the disadvantages of festoon ovens required in the manufacture of coated abrasives. U.S. Pat. No. 4,547,204 discloses the use of radiation curable acrylated epoxy resins in one adhesive layer of the coated abrasive and the use of a heat curable phenolic or acrylic latex resin in another adhesive layer of the coated abrasive. U.S. Pat. No. 4,588,419 discloses an adhesive for coated abrasives comprising a mixture of: (a) electron radiation curable resin system comprising an oligomer selected from the group consisting of urethane acrylates and epoxy acrylates, filler, and a diluent and (b) a heat curable resin selected from the group consisting of phenolic resins, melamine resins, amino resins, alkyd resins, and furan resins. U.S. Pat. No. 4,652,274 discloses a binder for coated abrasives, which can be cured by radiation energy, comprising a copolymer formed from an isocyanurate monomer having at least one pendant acrylate group and an aliphatic or cycloaliphatic monomer having at least one pendant acrylate group. U.S. Pat. No. 4,642,126 discloses a coated abrasive binder comprising diacrylated monomers, monofuctional monomers, acrylated oligomers, and a photoinitiator. U.S. Pat. No. 4,644,703 discloses a coated abrasive binder comprising diacrylated monomer, triacrylate monomers, and a photoinitiator. Although radiation curable binders solve the above-mentioned problems associated with thermally curable binders, with respect to a festoon oven, radiation curable binders generally are more expensive than thermally curable binders. In many abrasive products, this increase in cost cannot be tolerated; thus thermally curable resins are still utilized.